1. Field of Invention
This invention relates to a flame detector which detects the presence or absence of a flame by use of a light receiving device acting as the flame sensor.
2. Description of the Prior Art
One conventional type of flame detection system uses a mechanical shutter which is mounted between a light receiving device and the flame to provide a self checking function. Also, techniques are known for enhancing the reliability of the shutter mechanism, as described, for example, in Japan Laid Open Patent Nos. 122,530/1986 and 122,531/1986. In particular, in one known flame detector, a mechanical shutter, which is opened and closed at regular intervals of time, is disposed between the flame to be inspected and a flame sensor used to detect the presence or absence of the flame. In another known flame detector, a liquid crystal cell, which is switched between a transparent mode and an opaque mode at regular time intervals, is placed between the flame and the flame sensor. In a further known flame detector, a shutter cell, made of a material which transmits light, is placed between the the flame and the flame sensor. A magnetic fluid of a color which is capable of blocking light is sealed in the shutter cell, and magnets for controlling the position of the magnetic fluid inside the cell are disposed on opposite sides of the cell
In these conventional devices, the flame detector receives a given recurrent signal from the flame sensor. The detector has a fail-safe detector circuit which processes this signal in predetermined manner to constantly detect the presence or absence of a flame and of trouble in the flame sensor even during the operation of the device.
The conventional flame detectors of the type described have the following problems. (1) The structure of such devices requires an excessive number of components, such as the liquid crystal cell or the shutter cell. In addition, a component for controlling such excessive components, is required. This makes such prior art devices complex and bulky. Also, the devices are costly. (2) The liquid crystal cell and the shutter Cell are less likely to fail than the mechanical shutter. However, the failure rate is still unfavorably high, because such cells must be constantly switched ON and OFF. (3) When a mechanical shutter is used, it is not necessary to use any optical component, which itself is subject to leakage and aging, in the path between the flame and the flame sensor. However, when a liquid crystal cell or shutter cell is used, it is necessary to place the cell or like in the optical path between the flame and sensor. Leakage in the optical components and deterioration in transparency causes deterioration in detection sensitivity.
Thus, there is room in the art for improvements in flame detectors.